The 7G nano doesn't require the footpod, but it can utilize it. Here's why you might want to use it instead of the built-in pedometer.

Accuracy. The footpod is located on your shoe (or in it if you have the right Nike shoes). You're not going to get a more accurate placement than that. With the built-in pedometer, you really have to put it on your hip for best accuracy ... but then it's difficult to read while running. If you get an armband/wristband, then the accuracy may suffer due to your armswing.

To me, it's not contest. Get the footpod. It's only $20. A pouch goes for $5 on Amazon. It's worth it for the accuracy.

NOTE - Neither the footpod or the built-in pedometer will rival a GPS solution, but then again, GPS doesn't work on a treadmill or an indoor track.

NOTE 2 - there is more data collected when using the footpod. The footpod actually counts distance as well as pace. The built-in collects only pace, which the software will convert to distance ... at least that's how it worked on the 6G nano.

BTW, calories are calculated using a formula that takes into account your weight and distance traveled. Speed has very little to do with the calorie reported.